The 50-seat RJ fleet plan is kind of status-quo for the time being now that some of the regional providers have stabilized and are able to get pilots for the time being. In seems that in the short-term as long as they can continue to fly 50-seaters with economic service-life left they will use them. Beyond 2020, when every 50 seater out there is approaching 15-20+ years of service the cost of engine and airframe overhauls will render them uneconomical. However, the utilization (cycles and hours) per frame of the 50 seaters is way down from what it 5 years ago, and they've cycled some frames out of the desert as others have hit overhaul intervals.
Eventually this is going to start to catch-up to the CR7s which is an aircraft I've never really understood. I don't know how that airplane makes any economic sense which it has operating costs near the CRJ-900, carries less passengers, and is an inferior on-board product to the E170/E170 and even the CR9s for that matter.
Delta has no problem getting the RJ's staffed at OO and 9E. OO is able to staff via their diversity in flying and bases and 9E via pay and limited career progression. I think going forward you'll see a total of 3 regional carriers in DCI, OO, 9E and YX.
The operating costs of a CRJ7 is significantly lower than a 170. Add to that the limitation on large RJs preventing the whole fleet from being 76 seats, and the CR7 starts to make sense. Something is going to have to replace the 700, could it be new 700's or something else, who knows.